A technical problem to be solved in optical-fiber interconnection is power distribution from a transmission medium to a plurality of media while keeping the coupling uniform and the power loss low. In the fact, while the loss in case of a coupler between two fibers only is basically due to imperfect alignment and to different optical-fibers characteristics, the coupling loss between one and a plurality of fibers can be also due to shaded zones. Owing to these, the optical power outgoing from a fiber is not entirely collected by the other fibers.
In addition a good coupler should keep the original light beam characteristics unchanged above all with respect to the modal distribution.
In other words, different order modes propagating into fibers should not be mixed. Mixing may cause, e.g., a distortion of transmitted optical pulses, in the case of multimode fibers, and an excitation of non-propagating modes in case of monomode fibers.
As to fabrication, the design and mechanical parts ought to be as simple as possible and the number of different materials used should be limited.
Finally the coupler should be insensitive to external mechanical strains, which could give rise to misalignment and hence power loss.
An optical coupler is described in "Advanced in ceramics. II Vol. Physics of fiber optics" by Kapany, edited by "The American ceramic society". This coupler couples a fiber to two other fibers, by exploiting power reflection operated by two hemispherical surfaces. These surfaces concentrate the radiation emitted by a fiber into the other two fibers.
This coupler, however, has a number of disadvantages, including:
optical-signal distortion due to different lengths of the optical paths joining different fibers and the various points of hemispherical surfaces;
impossibility of coupling more than two fibers without significant modifications in the geometric shape of reflecting surfaces; and
sensitivity to mechanical vibrations as single fibers and reflecting surfaces undergo mutual oscillation.
In fact, to optimize the optical path it is necessary to leave a certain distance in the air between the fiber ends and the reflecting surfaces. As a consequence, the coupler can not be made in one piece.
Another coupling technique among a plurality of fibers consists of fusing a bundle of fibers duly prepared to obtain a zone where there is no core distinction. The fused zone is then drawn and possibly machined.
This coupler is difficult to make because of the preparation required for the single fibers, which must generally be deprived of their coatings, and in ensuring correct fusion of the whole.
In addition an inconvenience of this coupling is that it causes propagating modes to be mixed.